During petroleum drilling, water- or oil-based drilling mud compositions are used to remove drill cuttings from the well, to keep the drill bit cool and clean, and to provide hydrostatic pressure. Viscosifiers are often used in drilling muds. When forced against a permeable medium in the subterranean formation, the drilling mud can form a filter cake or mudcake, often containing salts and viscosifiers, that can block flow. Acid treatments and breakers such as esters, chelating agents, and oxidizers can be used to at least partially remove the mudcake prior to production to provide higher permeability of the formation. During hydraulic fracturing, high-viscosity materials can be used downhole to form fractures. The high-viscosity materials block flow channels in the formation, and can be removed from the formation to restore permeability, often using breakers to help degrade the viscous material for easier removal.
The thermal degradation of chemical additives in drilling muds that often occurs while drilling high temperature wells can be a major concern, leading to strong variations in rheological and filtration characteristics, as well as loss of fluid properties. One way to help remedy the negative effects of high downhole temperatures is to use formate-based mud (e.g., muds made using brines including formate salts such as sodium formate and potassium formate). Benefits of formate-based muds can include high solubility in water at a wide range of densities, an ability to extend the thermal stability of bio-polymers, and reduction of the rate of hydrolytic and oxidative degradation of many viscosifiers and fluid loss agents at high temperatures.
However, when it comes to removing formate based mud filter cake, many problems arise while using the conventional breakers like esters, chelating agents, and oxidizers. Formate brines are alkaline in nature and require a higher amount of acid to meet the pH required for dissolution of certain salts in the filter cake, such as carbonate salts. Formate brines have a tendency to buffer the pH of the breaker system and hence the salt dissolution efficiency (e.g., carbonate salt dissolution efficiency) can be reduced drastically. Being reducing in nature, formate brines stabilize the free radical and hence the viscosifiers are not removed effectively by oxidizer breakers. A mineral acid such as HCl with high acidity can be used to remove the filter cake, which can reverse the effect of alkalinity and buffering capacity of the formate brine. However, using mineral acid straight away can create an extreme drop in pH, which can lead to a pinhole effect and can cause an abrupt break of mudcake allowing huge fluid loss.